A design for the White House attributed to Thomas Jefferson, a replica of London’s Tower Bridge spanning the Potomac, a ziggurat-shaped Lincoln Memorial—these are among the stillborn designs included in Unbuilt Washington, an exhibition on display at the National Building Museum.The show presents drawings, models, and renderings of structures that either never made it off the boards, or did so in dramatically different fashion—all by architects that run the gamut from Pierre Charles L’Enfant to the New York–based firm Morphosis. In this slide show, see 10 of those plans, plus commentary drawn from writings by and conversations with the show’s curator, Martin Moellner. Through May 28. • nbm.org